1. Field of the Invention
The present invention relates to a communication device, and in particular to a communication device that detects a disconnection.
2. Description of Related Art
A USB (Universal Serial Bus) standard has been widely used as an interface to interconnect various media devices. Interfaces using the USB standard are incorporated in various sets depending on a medium to be used by users. A standard of a transmitting-waveform in a HS (High Speed) mode in the USB 2.0 standard is specified at each of a near-end of an output terminal side and a far-end of a receptacle terminal side through a transmission path. Then, Communication quality is ensured by guaranteeing these standards.
However, recently, the USB standard is incorporated in devices for various use, and the usage of the USB standard has been diversified. As a result, the devices incorporating the USB standard have been interconnected in many cases without involving a personal computer. Therefore, in order to achieve a high communication quality adapted to various usage environments, more users wish to guarantee a transmitting-waveform in a near-end standard on a receptacle side. Accordingly, there is an increasing demand for developing a device that can perform a communication that satisfies the near-end standard, when a waveform loss due to an effect of a transmission path between an interface communication device and a receptacle in a set incorporating the USB standard is taken into consideration.
Here, the USB standard is described. A USB is a two-wire communication bus. The USB 2.0 standard is a communication standard which supports three transmission modes of High Speed (HS), Full Speed (FS), and Low Speed (LS) (Universal Serial Bus Specification Revision 2.0 pp. 119-194). The USB 2.0 standard has a specification that includes a HS transmitting circuit, a squelch detector circuit; and a disconnection detector circuit. The HS transmitting circuit outputs the transmitting-waveform when a HS communication is performed. The squelch detector circuit detects a received signal intensity. The disconnection detector circuit detects that an opposite device is opened. When the output level is 360 mV to 440 mV, the detection sensitivity of the squelch is specified in the range of 100 mV to 150 mV and the detection sensitivity of the disconnection is specified in the range of 525 mV to 625 mV.
FIG. 7 is a block diagram showing a connection between a transmitting-side device and a receiving-side device in a HS mode of the USB 2.0 standard. Signals DP and DM constituting a pair of differential signals are terminated at 45Ω in each of the transmitting-side device and the receiving-side device. The HS transmitting circuit is specified to drive a current which makes a differential signal amplitude be about 400 mV when a combined resistance between the transmitting-side and the receiving-side is 22.5Ω. Further, when the receiving-side device is removed, the differential signal amplitude becomes 800 mV which is about twice as large as that described above. Disconnection detection is made to judge whether the device is removed or not by detecting a differential amplitude level. A disconnection detector circuit 70 detects the differential signal amplitude between the signals DP and DM on the output side of the HS transmitting circuit, and judges whether the device is removed or not based on the intensity of the differential signal amplitude.
FIG. 8 is a graph showing a general waveform when a SOF (Start of Frame) packet is transmitted in the HS mode of the USB 2.0 standard. A horizontal axis in FIG. 8 represents time. A vertical axis in FIG. 8 represents a voltage level that is differentially amplified. The packet includes a SYNC pattern of 55 to 115 nsec and an EOP (End of Packet) pattern of 175 to 255 nsec. In this way, the differential amplitude level greatly varies between a minimum inversion span such as the SYNC pattern (frequency of 240 MHz) and a non-inversion span such as the EOP pattern (frequency of 6 MHz) depending on frequency characteristics of an amplifier. For this reason, the USB 2.0 standard is specified such that the disconnection detection is performed in the EOP pattern in which the amplitude is stable.
FIG. 9 is a circuit diagram of a disconnection detector circuit 200 in the USB 2.0 standard disclosed in Japanese Unexamined Patent Application Publication No. 2009-65235. A voltage difference between the signals DP and DM is amplified by a first differential amplifier 21a in the disconnection detector circuit 200. The amplified voltage difference is output to a subtractor 22a. A voltage difference between the signals DM and DP is amplified by a second differential amplifier 21b. The amplified voltage difference is output to a subtractor 22b. A DA converter 26 outputs a reference voltage based on input data (predetermined value DAC Data). The reference voltage is amplified by a third differential amplifier 21c (a circuit manufactured by so-called “replica” method) having the same characteristics as those of the first and second amplifiers. The amplified voltage is output to the subtractors 22a and 22b. The subtractor 22a subtracts the output of the differential amplifier 21c from the output of the differential amplifier 21a, and outputs the subtracted output to a zero-cross comparator 23a. The subtractor 22b subtracts the output of the differential amplifier 21c from the output of the differential amplifier 21b, and outputs the subtracted output to a zero-cross comparator 23b. An OR circuit 24 performs an OR operation on the outputs of the zero-cross comparators 23a and 23b. The output of the OR circuit 24 is latched based on a latch timing signal LT by a latch 25 and output as a disconnection detecting signal LO. The latch timing signal LT is output at the end of the EOP span of the SOF packet.